[1] Univ Manchester, Sch Epidemiol & Hlth Sci, Unit Chron Dis Epidemiol & Hlth Sci, Manchester M13 9PT, Lancs, England
[2] Univ London London Sch Hyg & Trop Med, London WC1E 7HT, England
[3] Guys Kings & St Thomass Sch Med, Dept Psychol Med, Gulf War Illnesses Res Unit, London SE5 8AZ, England
[4] Univ Manchester, Sch Epidemiol & Hlth Sci, Epidemiol Unit, Arthrit Res Campaign, Manchester M13 9PL, Lancs, England